The statement, often attributed to William Morris Davis, encapsulates his influential 'Cycle of Erosion' theory, which posits that landforms evolve through a predictable sequence over time. In this model, 'structure' refers to the geological characteristics of the region, including rock type, folds, faults, and their resistance to erosion. 'Process' denotes the geomorphic agents acting on the landscape, such as weathering, erosion (by rivers, glaciers, wind), and mass wasting. 'Stage' represents the temporal evolution of the landscape, typically categorized into youth, maturity, and old age, each with distinct landform assemblages.

Critique: While Davis's model provided a foundational framework for understanding landscape evolution and introduced systematic geomorphological analysis, it faces several significant criticisms:

1. Oversimplification: The model assumes a rapid, instantaneous uplift followed by a prolonged period of stability during which erosion proceeds. In reality, uplift and erosion often occur simultaneously and intermittently, leading to more complex interactions.
2. Deterministic and Unilinear: It suggests a fixed, unilinear progression through stages, implying that all landscapes will eventually reach a peneplain (old age). This deterministic view does not account for variations caused by different climatic conditions, tectonic activities, or human interventions.
3. Ignores Climatic Changes: Davis's model largely assumes a constant climate throughout the cycle. However, climate is a dynamic factor that significantly influences geomorphic processes and can alter the pace and nature of landscape development.
4. Lack of Quantitative Basis: The stages are qualitative and descriptive, lacking precise quantitative measures for defining transitions between youth, maturity, and old age. This makes it difficult to apply universally or to test empirically.
5. Emphasis on Endogenic Factors: While acknowledging structure, the model primarily focuses on exogenic processes. It does not fully integrate the continuous interplay between tectonic forces (uplift) and erosional processes.
6. Ignores Human Impact: The model predates significant human alteration of landscapes, which now plays a crucial role in shaping landforms and accelerating or modifying natural processes.

Despite these criticisms, Davis's cycle of erosion remains important for its conceptual clarity and its role in establishing geomorphology as a systematic science. It provided a valuable starting point for subsequent theories and continues to be a useful pedagogical tool for introducing fundamental concepts of landscape evolution, even as modern geomorphology incorporates more dynamic, quantitative, and process-oriented approaches.